BNEN31 lumbar spine back x-ray of a 55-year-old woman showing a spinal fusion. Image shot in 2010. [Image courtesy of Foster Corp.]
Polymers have always played a role in modern implantable medical devices. But until the late 1990s, many suppliers limited availability for long-term implants due to potential litigation liability in an inherently low-material-volume market. As such, metals remained the materials of choice through the end of the century.
Several factors changed the implantable polymer paradigm, including passage of the Biomaterials Access Assurance Act in 1998, which limited the civil liability of material suppliers. Suppliers also began developing business models to provide adequate returns on low-volume sales. As a result, polymer supply and innovation for implantable medical devices surged in the 21stcentury. No polymer represents this dynamic growth more than PEEK – polyether ether ketone.
In less than two decades, PEEK has become a material of choice for some orthopedic implants, such as intervertebral fusion cages. This has been attributed, in part, to the anatomical compatibility of PEEK and inherent radiolucency of the polymer.
Many traditional orthopedic implants, including fusion cages, were manufactured from titanium and stainless steel. These materials are extremely rigid, with elastic moduli of 100 GPa (14,500 ksi) and 193 GPa (28,000 ksi), respectively. By comparison, the elastic modulus of cortical bone is approximately 18 GPa (2,610 ksi). This disparity in rigidity can cause stress concentration on the skeletal structure adjacent to the implant.
In contrast, unreinforced PEEK has an elastic modulus of 3.7 GPa (540 ksi) and a tensile elongation of 45%. This provides a semi-rigid, yet tough implant that reduces stress concentration on adjacent bone. Implants have also been developed that are made of PEEK compounds reinforced with chopped carbon fiber, resulting in device rigidity equal to that of bone for greater structural continuity.
Furthermore, metal implants are inherently opaque to X-rays (radiopaque), which limits the postoperative visibility necessary for some procedures. A major advantage of PEEK over metal intervertebral spinal fusion cages is inherent X-ray transparency (radiolucency) for postoperative evaluations.
Success in intervertebral cages led to increased interest in PEEK for other applications, including orthopedic trauma fixation, spinal stabilization, dental implants and ligament anchors. Whereas the inherent radiolucency of PEEK is suitable for spinal fusion procedures, some of these new devices must be radiopaque.
Radiopaque fillers can be compounded into PEEK to enhance x-ray visibility. Selection of the appropriate filler and loading is based on clinical requirements, component size and location within the body. The most common additives used to enhance the radiopacity of medical polymers are bismuth subcarbonate, bismuth trioxide, bismuth oxychloride and barium sulfate. The high melt temperature of PEEK, which exceeds 335oC (635oF), limits several of these radiopaque filler options.
Bismuth subcarbonate is limited to melt processing temperatures up to 205oF (400oC), beyond which it becomes unstable and turns from white to yellow; bismuth trioxide tends to turn from white to brown at PEEK processing temperatures. Bismuth oxychloride can be processed at elevated temperatures, but it reduces the melt flow of PEEK, which can impact molding or extruding quality parts.
Barium sulfate is the most common additive for radiopaque PEEK formulations. It disperses well in the polymer and can withstand the processing temperatures without color change. Barium sulfate is also biocompatible and has been used in minimally invasive devices (<30 days) and long-term implants (>30 days), such as bone cements.
As PEEK continues to expand into new temporary and long-term implantable devices, the availability of custom formulations designed to enhance key properties will expand. At the forefront of consideration for suitable additives will be the elevated compounding and component processing temperatures of the polymer.